The oblong cross section of the tubes may depart to a greater or lesser extent from the shape of a circle, and it may for example, be oval, elliptical or flattened in shape. In this last case it may have two long sides opposed to each other, and two opposed short sides. The long sides may be straight or curved and the short sides are curved. Such tubes of oblong cross section are so disposed that the largest dimension of their cross section extends in the direction which is substantially parallel to the direction of flow of air through the bundle of tubes in the heat exchanger. In this way, heat exchangers are made which have both an improved efficiency and a reduced size as compared with heat exchangers in which the tubes are of circular cross section.
In heat exchangers of the type described above, which may also be called "mechanical assembly heat exchangers", the quality of the seal at the level of the joint between the end portion of each tube and the header plate depends mainly on the mechanical strength of the tube end portion when subjected to the mechanical force exerted on it by the surrounding portion of the sealing gasket. The resistance of the tube end portion to this applied force depends mainly on its rigidity, and especially on the shape of its cross section. In this connection, the closer the cross sectional shape of the end portion is to a circle, the greater will be its rigidity. Conversely, the more the shape of the cross section departs from a circular shape, i.e. the more flattened it is, the less rigid will be the end portion of the tube. This reduction in rigidity occurs in the regions of the cross section which lie in the direction of the largest dimension, that is to say along its long sides. This represents a drawback which is all the more serious in view of the current trend towards the use of tubes having an oblong cross section with a very flattened configuration.
In order to overcome this disadvantage, various solutions have already been proposed for increasing the strength of the end portions of tubes having oblong cross sections. For example, one such solution is described in the specification of French published patent application FR 2406794A, which proposes, in particular, that the long sides of the oblong cross section should be thickened as compared with the short sides. This makes it necessary to extrude special tubes of non-constant thickness. The same French patent specification also proposes to introduce particular special profile shapes into the end portions of the tube, such as to prevent deformation of the end portions. Such a solution is not satisfactory because it necessitates an additional fitting operation, and it is also detrimental to the flow of fluid through the tubes.
In addition, it has been proposed, for example in the specification of French published patent application FR 2570814A, to form a peripheral bead on the end portion of a tube of circular cross section, so as to serve as an abutment during introduction of this end portion into a corresponding hole in a header plate. In this way a predetermined spacing is provided between two header plates, and therefore between two headers of a heat exchanger.